I have a string of 0s and 1s of some length (lets say "1010101011"), and a list of indices (lets say 2, 5, 8). I should return TRUE if the bits at these indices are the negation of the bits before them. In our example:

Bit 2 is NOT bit 1, so OK. (indices range 0 to string length - 1).
Bit 5 is NOT bit 4, so OK.
Bit 8 is NOT bit 7, so OK.
All 3 OK, so I return TRUE. Had one of these been false, I'd return FALSE.

Pretty simple, but I must make it as fast as possible. The basic approach would be just a loop on these indices and checking with substr() for each of them whether it's the NOT of the previous. Or a split and then array operations (I've yet to time them against each other).

Is there a quicker way ? Remember: the input is a string "..." and a list of indices into this string. For simplicity sake assume correctness of all arguments.